Reproductive physiology of poultry
Managing the reproductive processes of the hen and rooster is the basis of broiler breeder production. An understanding of the basic reproductive physiology of both sexes is important in applying management principles involving feeding, lighting and health management. Following is an overview of reproductive processes in both sexes which are to some extent subject to manipulation by the breeder manager.
STRUCTURE OF THE OVARY AND OVIDUCT
The mature hen has only one ovary and oviduct even though left and right reproductive systems are evident during very early incubation. Unlike the situation with testes in the male, the right ovary and oviduct regress during mid-incubation, and are nonfunctional in all “normal” hens.
The left ovary is found deep in the body cavity lying in close proximity to the left kidney. During incubation as many as 20,000 eggs develop, of which about 2,000 are visible to the naked eye. There should be minimal development of these oocytes in the growing pullet, and consequently the ovary should be fairly difficult to find during necropsy. During maturation, a hierarchy of ovum will develop so as to supply a sequence of eggs for daily ovulation. In the mature hen the ovary should weigh around 35g, being composed of 3-4 large “maturing” follicles, and a series of 8-12 follicles of ever diminishing size.
The follicles consist of concentric layers of “yolk” that are continually being deposited. If fat soluble dyes of different color are given to the hen daily, then daily concentric deposits are clearly visible if the follicle is bisected. The greatest mass of the follicle, which comprises about 50% fat and 50% protein per unit of dry matter, is deposited in the last 3-4d prior to the follicle being ovulated.
If follicles fail to ovulate, then they start to regress which can involve gradual re-absorption or rupture and loss of contents into the body cavity which itself can contribute to peritonitis. Such regression of the follicle is most commonly caused by molting or under feeding.
The oviduct is not attached to the ovary, and so when follicles are released they must “fall” into the funnel shaped opening of the oviduct called the infundibulum. Follicles that fail to reach the oviduct are lost into the body cavity which most commonly occurs with multiple ovulations (see figure) which unfortunately is a more common occurrence in broiler breeders rather than Leghorn hens.
The infundibulum also contains sperm storage glands. Albumen is formed in the next region of the oviduct known as the magnum while the shell membranes are deposited in the mid isthmus region. The egg spends the majority of its time in the shell gland, where over 15-18 hours the shell material is gradually deposited. The shell pigments are finally deposited during the last 2-3h before egg laying.
OVULATION
Ideally, breeders will lay eggs on a consistent cycle with a minimum of non-laying days. At peak production, we routinely see flocks laying at 85-87%, and periodically individual flocks reach 90% production for 10-14d. This sustained high rate of production means that birds are ovulating close to a 24h cycle. The release of the follicle from the ovary is controlled by hormones produced within the pituitary and the follicles themselves, and both are under control of the lighting schedule. As the largest, or so-called F1 follicle, in the ovary “matures”, its production of hormones changes resulting in greater output of progesterone. This change itself is controlled by output of luteinising hormone (LH) and follicle stimulating hormone (FSH) from the brain, which is controlled by the light/ dark cycle. The release of luteinising hormone from the brain only occurs during a 6-8h period each day, and this is influenced by the pattern of light:dark each day. It is the failure to release luteinizing hormone in this “window” that results in a non-egg laying day, or a pause in the clutch sequence.
EFFECT OF FEEDING LEVEL ON OVULATION
Unfortunately the broiler breeder sometimes exhibits the very undesirable characteristic of multiple ovulation. Instead of releasing just one follicle per day, she can release 2 and sometimes 3 or 4 follicles per day. This trait of multiple ovulation is usually in response to overfeeding, and especially when there is an excess of energy. Multiple ovulation therefore, often precedes or accompanies obesity in breeder hens. When more than one follicle is released, there can be double or even triple yolked eggs produced, although more commonly the extra follicles end up in the body cavity. The practical consequences of multiple ovulation are either the production of unsettable double yolked eggs, or an erratic pattern of egg laying, because of so many lost follicles. For example, with two follicles released at one time, the production rate can be no greater than 50%. When broiler breeders are overfed, they develop more large follicles and in extreme cases there may be two or three replicate hierarchies present in birds fed close to ad-libitum. Even overfeeding adult birds as little as 10%, can lead to increase in multiple ovulations. Apart from there being a net loss of settable eggs, any normal eggs that are produced often appear at unusual times of the day because of erratic times of ovulation.
BROODINESS AND MOLTING
Broodiness involves the natural instinct of the hen to stop ovulating and to incubate her eggs. This involves behavioral changes together with major interruption of feeding and drinking. Characteristically, hens resist being moved from the nest exhibiting raised neck feathers and clucking when approached. Fortunately broodiness is a trait of moderately high heritability, and so over time the characteristic has been selected against in breeding programs. While Leghorn birds rarely show broodiness, it is sometimes seen in broiler breeders in moderately warm climates and especially in male line grandparents. As levels of luteinizing hormone decline, so levels of prolactin increase, and in the turkey at least, there is a direct relationship between levels of prolactin and duration of broodiness. Since birds rarely exhibit broodiness in cages, then it is obvious that environment must play a major role in stimulating this behavior in hens. Broody hens seek out a nest site that is isolated, dark, warm, moist and, ideally, already has a number of eggs present.
Providing such environments in a commercial breeder house is therefore simply poor management. Broodiness can be stopped by moving the hens to a new environment, and especially one that has a different floor material. However, even under the best broody management systems, it will take some 15-21d for resumption of egg production, and during this time of rehabilitation, it is time consuming to feed birds according to their changing nutrient needs. The best broody management is prevention by timely collection of nest and floor eggs and to exclude “ideal” nesting sites on the litter.
MALE REPRODUCTIVE SYSTEM
With natural mating, the function of the male is to produce viable sperm in the testes and then to efficiently transport these to the cloaca of the hen during mating. Unlike the situation in most mammals, the testes of the rooster are deep within the body cavity and so function at the normal body temperature of close to 41°C. The testes are found close to the kidneys. The testes are quite small prior to maturity, being only 1-2g each, although similar to the situation with the ovary in the hen, there is a dramatic increase in size starting at around 18 weeks, and the mature testes are 15-20g and easily visible.
There is a direct relationship between sperm production and testes size, and the latter to some extent is positively correlated with body size. Daily sperm production is about 100 million per gram of testes weight. Sperm production is fairly constant regardless of mating or collection frequency. Therefore, with higher frequency of mating, there will be fewer sperm per ejaculate. If ejaculation does not occur over a 2-3d period, then any sperm stored in the vas deferens are reabsorbed.
During mating, or collection during artificial insemination, the semen is directed from the vas deferens to the engorged phallic folds on the cloaca. Such engorgement is caused by lymphatic fluid. The semen is usually white and opaque, but can be clear and watery which a sign of reduced sperm concentration is often. The hormone testosterone is produced by the testes under regulation of gonadotrophins, and similar to the female system, overall control of the breeding cycle is ultimately dictated by the photoperiod. Comb size is a sensitive indicator of testosterone level, and so can be used to evaluate stage of maturity in developing roosters. Collection of 4-5 semen samples over a 7-10d period is sufficient to predict a rooster’s potential life-time fertility.
